Carbon Kinetic Isotope Effects Reveal Variations in Reactivity of Intermediates in the Formation of Protonated Carbonic Acid

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• 作者：Adelle A. Vandersteen ; Scott O. C. Mundle ; Georges Lacrampe-Couloume ; Barbara Sherwood Lollar ; Ronald Kluger
• 刊名：Journal of Organic Chemistry
• 出版年：2013
• 出版时间：December 6, 2013
• 年：2013
• 卷：78
• 期：23
• 页码：12176-12181
• 全文大小：302K
• 年卷期：v.78,no.23(December 6, 2013)
• ISSN：1520-6904

文摘

Kinetic evidence suggests that acid-catalyzed decarboxylation reactions of aromatic carboxylic acids can occur by a hydrolytic process that generates protonated carbonic acid (PCA) as the precursor of CO₂. Measurements of reaction rates and carbon kinetic isotope effects (CKIE) for decarboxylation of isomeric sets of heterocyclic carboxylic acids in acidic solutions reveal that C鈥揅 cleavage to form PCA is rate-determining with significant variation in the magnitude of the observed CKIE (1.018鈥?.043). Larger values are associated with the more reactive member in each isomeric pair. This variation is consistent with stepwise mechanisms in which C鈥揅 cleavage is competitive with C鈥揙 cleavage, leading to reversion to the protonated reactant to varying degrees with an invariant intrinsic CKIE for C鈥揅 cleavage. Thus, the relative barriers to reversion and formation of PCA control the magnitude of the observed CKIE in a predictable manner that correlates with reactivity. Application of the proposed overall mechanism reveals that carboxylation reactions in acidic solutions will proceed by way of initial formation of PCA.